The Seepage Problem and Remediation at Fern Ridge Dam

Fern Ridge Dam, near Eugene, Oregon, was constructed in 1940, the first of thirteen earth dams in the Portland District of the U.S. Army Corps of Engineers. The embankment dam is over 60 years old and was built without the benefit of modern filter criteria. There were indications that the existing filter was too coarse and that the corrugated metal drainpipe may have been deteriorating. From 2002 to 2004, a series of observations and investigations of seepage conditions indicated that internal erosion (piping) into the dam's drainage system was occurring. In addition to turbid flows during rain events, silts and sands were observed in seepage flow during sustained pool levels with no rain. The worst-case scenario was that erosion of the foundation could lead to either a piping failure or clogging of the drain. With the dam in an active state of piping failure, a pool restriction with monitoring was implemented. In February 2005, the Corps decided to start an expedited repair of the dam's drainage system in order to return the project to full operation by the following fall. Preparation of plans and specifications began immediately and the dam was repaired and fully operational by November 2005. This paper describes the observations and investigations leading to the repair, the decision to repair the dam, the repair itself, the evidence of erosion found during the repair, and the post-repair performance to date. The case history may be useful to engineers and dam owners facing difficult repair decisions. 13 pp., 6 references.